Integrated Wavelength Control for Diode-Pumped Lasers

Dr. Robert S. Williamson III, Alfalight Inc.

Diode-pumped solid-state laser systems are replacing flashlamp-pumped lasers in many applications because diode pumping results in better overall wall-plug efficiency and beam quality than lamp pumping. Furthermore, diode pumping is the only feasible approach to energizing recently commercialized architectures such as thin-disk and fiber lasers.

Driven by ever-increasing commercial demand and by government initiatives such as the DARPA SHEDS program and the Air Force LADERA program, the performance of semiconductor pump lasers has continually improved in terms of energy efficiency, available output power and overall package reliability.

This article will describe a unique approach to further improvement of pump diode laser performance. By adding a monolithic semiconductor grating to a multimode semiconductor laser cavity, we have not only improved the spectral match between it and the rare-earth-doped gain medium it pumps but also stabilized that match against changes in the diode’s operating temperature.

Spectral characteristics

The good spectral match between a diode laser’s output and the absorption of a rare-earth-doped crystal is a substantial reason for diode pumping. The spectra of rare-earth-doped crystals and fibers typically have only a few absorption features, each only a few nanometers wide (Figure 1). A lamp with a spectrum typically greater than 100 nm pumps the entire manifold of absorption lines but generates much light that is not absorbed at all. The result is low wall-plug efficiency for the lamp-pumped laser.

Figure 1. These plots show the relative absorption spectra of (left) Yb-doped glass fiber and (right) Nd:YAG gain crystal in a 100-nm window. The spectra have strong, narrow features that are typically only 3 to 6 nm wide.

The high-performance broad-area lasers used in diode-pumped solid-state laser pumping typically have a spectral width of 2 to 4 nm full width half maximum (FWHM), so that nearly all their output can be absorbed in the optimal rare-earth absorption line. To achieve this absorption, however, pump lasers must be manufactured with a tight wavelength tolerance, usually maintaining the center wavelength within a few nanometers. And their inherent wavelength shift with temperature — typically 0.3 to 0.35 nm/°C — requires thermal control.

The gold standard for thermal control is the thermoelectric cooler, which can easily hold temperatures well within 0.1 °C. However, this control adds complexity and increases power consumption. Thermoelectric coolers typically consume one to four times as much power as the heat they remove. Furthermore, they require a controller, power supply and temperature sensor.

Simpler methods that provide less temperature control but are sufficient in some cases include water cooling or a thermostatically controlled fan. However, the application of these cooling methods is limited. Not all systems can accommodate the manifolds, mechanical pumps, heat exchangers and maintenance associated with water cooling. Fan cooling is simpler but has limited capacity and typically requires the laser to operate well above ambient temperature to provide control head room, leading to reduced diode lifetime.

Optical control

A more elegant approach is to add an optical element to control the spectral properties of the diode so that the output is less dependent on temperature. As with any laser, a diode laser depends on the cavity mirrors and gain medium properties to determine the output spectrum. External cavities — that is, a laser resonator based on external optical components and not solely on reflections from the diode’s facets — have been used to control the spectra of single-mode diode lasers for more than two decades. Fiber Bragg gratings, bulk gratings and other components provide feedback only at selected wavelengths, forcing the laser to emit at a single wavelength dictated by the grating pitch.

But only recently have these methods made their way to multimode high-power pump diodes. How do you provide useful optical feedback in an inherently multimode laser system? The methods are quite similar. They use an external or an internal grating structure, but implementation has required the right level of development effort to make them practical. The result is a significant improvement in both spectral width and wavelength temperature stability.

External cavity methods involve using a separate optical element to provide optical feedback to the diode laser. In a fiber Bragg grating, an optical grating is written into the core of a photosensitized fiber using UV light, usually in a holographic arrangement. The resulting grating is typically a small modulation in the index of the glass core. A fiber with a few centimeters of this grating, placed near the output facet of the laser diode, provides optical feedback to the diode. Only certain wavelengths of light are reflected from the fiber grating back into the laser, selectively reinforcing only those wavelengths.

A volume Bragg grating is similar in that it is implemented externally to the diode laser chip, but it uses a transmissive bulk optical element placed at the output of the laser diode rather than a fiber (Figure 2). Similar to the fiber Bragg grating, the grating in the glass selectively reflects a limited range of wavelengths, forming an external cavity that narrows the wavelength of the diode.

Figure 2. The laser diode, emitting a wide spectrum (represented as several wavelengths λ_1...N), impinges on the volume Bragg grating. The grating selectively reflects only one wavelength (λ₁) back into the laser chip, acting as a selective external cavity mirror to control the laser spectrum. The selection leads to the laser’s emitting just one wavelength λ₁.

These techniques narrow the output spectrum of the laser and reduce the wavelength drift with temperature considerably, delivering typically 0.1- to 0.5-nm linewidth and 0.01 nm/°C of thermal drift. A drawback of these techniques is the complexity and mechanical delicacy inherent in the addition of extracavity optical elements.

Our goal has been to provide the additional feedback control within a broad-area multimode pump laser without adding extracavity optical elements.

To accomplish this, we added a holographically defined optical grating to the standard diode-laser structure. It is integrated at the wafer level and implemented using the semiconductor material itself (Figure 3). The laser-active region, or quantum well, is shown as the thin red layer in the figure; this is where photons are generated from injected current. The laser optical cavity is formed by diode facets acting as cavity mirrors, which are coated for high (left) and moderate (right) reflection. The grating is well outside the active region but close enough to interact with the beam propagating in the resonator, whose profile is shown in blue.

Figure 3. This cross-sectional view shows a laser diode with Alfalight’s integrated wavelength stabilization technology. A semiconductor grating, an integral part of the diode manufactured at the wafer level, provides feedback into the laser cavity by interacting with the tail of the optical intensity profile inside the cavity (blue). The grating selects and reinforces only a few of the laser’s Fabry-Perot modes, narrowing and stabilizing the diode output spectrum.

The grating acts as a filter that selectively reinforces laser light that diffracts from the grating but that provides no feedback for laser light that is not resonant with the grating. As a result, only a handful (two or three) of the laser’s normal Fabry-Perot modes oscillate, and the laser’s spectral width is reduced to approximately 0.3 nm, a tenfold improvement over a traditional pump laser without the grating. Note that the grating acts on the longitudinal modes only, and the laser still oscillates in multiple transverse modes.

The grating also reduces the rate of wavelength change with temperature. In a normal diode, the wavelength change of ~0.3 nm/°C is caused by thermal shifts in the quantum well material itself. In a stabilized diode, the shift is controlled by the mechanical thermal expansion and contraction of the grating, reducing the thermal drift to ~0.07 nm/°C, a reduction of roughly five times.

Figure 4 compares the measured spectra of a 976-nm broad-area diode laser with and without integrated grating wavelength-stabilization technology. Without the grating, the output drifts about 14 nm over a 40 °C operating range. With the grating, the laser drifts only 3.5 nm over the same temperature range. Moreover, the linewidth of the laser with the grating is an order of magnitude narrower than without it.

Figure 4. This plot shows the optical spectra of both a traditional broad-area laser and a grating-stabilized laser over operating temperatures from 10 to 50 °C. The dotted lines show how the traditional laser is broad (3 nm FWHM) and how it shifts by 14 nm over a 40 °C change. The solid lines show the narrow (0.3 nm FWHM) grating-stabilized spectrum shifting by only 3.5 nm over 40 °C.

Because the grating is integrated monolithically into the laser structure, this approach suffers none of the mechanical difficulties associated with extracavity optical elements. The technique applies to both single-stripe emitters as well as bar arrays, and devices are packaged in exactly the same manner as standard pump diodes, with no change to the mounting or assembly processes.

At the system level, it is important that the wavelength control remain robust across a foreseeable range of operating conditions. To remain locked, the wavelength control mechanism requires that the emission spectrum of the gain medium and the grating pitch overlap sufficiently.

Figure 5. Performance of a single-emitter 976-nm grating-stabilized laser diode over operating temperature and drive current is shown. The green polygon indicates the range of temperature and drive currents for which the entire diode spectrum remains fully locked: All of the power is in a very narrow spectrum in this region. Outside the green box, the laser transitions gently from full lock to fractional lock. The device remains locked over a 40 °C window and from 0.1 to above 2.5 W of output power.

Figure 5 shows a parametric map of case temperature and operating current for a wavelength-stabilized diode laser. Inside the green polygon, the output spectrum of the pump is fully locked, with 100 percent of the power in a narrow peak, as shown in Figure 4. This window is 40 °C wide and extends across the full power range of the device, from 0.1 to beyond 2.5 W. The sloped left and right margins reflect the thermal junction resistance of the laser and package, which, in this case, is about 4 °C/W.

Outside the green locked polygon, the laser light slowly transitions from completely locked to unlocked over a ~15 °C range. In the gray unlocked region, the laser reverts to behaving like a traditional broad-area laser. Note that the total output power from the laser remains constant through the locked to unlocked transition.

Figure 6. This figure shows the calculated pump absorption from a 976-nm pump laser into an Yb-glass laser, as a function of operating temperature. The output spectrum of the pump diode is convolved with the Yb absorption spectrum to yield total pump absorption versus temperature. The relative absorption of a traditional broad-area laser shows strong absorption at nominal temperature (35 °C in this example) but falls off substantially at lower and higher temperatures, illustrating the need for thermal control of nonstabilized pumps (A). The absorption of a wavelength-stabilized pump shows relatively flat absorption over a wide operating temperature range, resulting from both the narrow and temperature-stable pump spectrum (B).

Figure 6 provides an example of the potential impact of wavelength-stabilized pump lasers on a diode-pumped solid-state laser. Here we show the absorption of a 976-nm pump diode in an Yb-doped glass fiber. The output from the pump diode is convolved with the gain fiber absorption spectrum to provide a measure of the total pump power absorbed as a function of temperature.

Figure 6A shows the relative absorption of a standard non-stabilized pump laser as the diode temperature is varied; Figure 6B shows the absorption of an internal grating stabilized pump. In this example, the relative absorption of the stabilized laser is both stronger and flatter, providing a wider useful temperature range. Using a metric of full width at 80 percent of maximum absorption, the stabilized pump offers an operating window in excess of 40 °C, whereas the traditional broad-area laser delivers an operating window narrower than 10 °C.

Acknowledgment

The author would like to thank the Air Force Research Laboratory for its financial support toward development of integrated wavelength stabilization technology.

Meet the author

Robert S. Williamson III is director of marketing and business development at Alfalight Inc. in Madison, Wis.; e-mail: rswiii@alfalight.com.